Below are some basic troubleshooting tips for common problems. If you try the tips below and still have issues, try asking on the Discord server for help.
Drivers
Ensure that you are using the latest reference drivers available for your video card. Often drivers that shipped with the card on a CD or came stock with the system have issues/bugs.
Choosing a Rendering Backend
For general compatibility, OpenGL is generally advisable; however, some cards run better under Direct3D. Direct3D is good on systems with limited memory or weaker hardware.
You can try both to see which one works better on your system. If you own a Voodoo Graphics card, the Glide backend will be the most performant one for you (unless you need to play in a window).
A (loose) suggested renderer list by vendor:
Nvidia / ATI: OpenGL
3dfx: Glide
S3 / Matrox / Other: Direct3D
Configuration
All settings can be manually modified in settings.ini located in the application's configuration folder. This can be quickly found using the setup tool and opening View -> Config Folder. Some settings can only be tweaked there.
Choose a resolution that matches your desktop resolution. Avoiding the resolution change can prevent problems with buggy video drivers. If this still fails, try a small well-known resolution like 640x480 or 800x600.
It's generally best to avoid manually setting the bit depth/refresh rate. If required, these can be configured via DISPLAY_BITDEPTH and DISPLAY_REFRESH_RATE under the [Display] section of settings.ini.